A number of pumps for infusion of medical solutions to patients have been developed over the years. Without exception, it is necessary and desirable that pumped medical solutions not enter into direct contact with the internal components of the pump. This is so either to prevent contamination of the solution, or to prevent corrosion of the pump caused by a medical solution.
One device which does not require direct contact between the internal mechanisms of the pump and the pumped fluid is the well-known peristaltic pump. A peristaltic pump is a type of pump which uses wave-like motion against the walls of a flexible tube that contains the fluid to be pumped in order to pump the fluid. As is well known, peristaltic pumps may be of two varieties: rotary or linear. Linear peristaltic pumps are preferred over rotary peristaltic pumps for certain applications because they possess certain advantages over rotary peristaltic pumps. In particular, some of the advantages associated with the linear type include operationally lower shear and tensile stresses imposed on the tubing which is used to convey the fluid. Also, there is less tendency toward spallation of the tubing's inner walls. Additionally, linear peristaltic pumps impose relatively lower forces on the tubing than do most rotary peristaltic pumps. This is important because the pumped fluid may be damaged when relatively high forces are imposed on the tubing.
Linear peristaltic pumps achieve these relative advantages by using reciprocating parts to provide peristaltic action against the tube to move the fluid through the tube. More specifically, linear pumps typically use a plurality of reciprocating fingers that are sequentially urged against the tube, which in turn causes sequential occlusion of adjacent segments of the tube in a wave-like action. Ideally, the speed with which the reciprocating fingers move toward the tube during a pump stroke is not constant. This is so because, as the tubing is squeezed, equal increments of finger motion produce progressively larger displacements of fluid. The ideal finger motion is therefore relatively rapid at the start of the stroke and then slower as the stroke progresses. It will be appreciated that the benefit of the ideal variable finger speed motion described above is to provide a uniform rate of fluid delivery over the stroke cycle.
Obtaining variable finger speed in linear peristaltic pumps, however, is not without its costs. This is so because the drive mechanism which actuates the fingers must account for a load which varies as finger speed varies. Conventional drive mechanisms have accounted for variable actuator load by simply imposing the variable load on the actuator motor. The skilled artisan will recognize that because the motor used in a drive mechanism must be sized to account for peak load, rather than average load, this method of allocating load variations requires the use of relatively large motors. Furthermore, it is generally true that when variable loads are imposed on motors, the useful life of the motors tends to be reduced. In addition, as is well known in the art, a motor that produces work at a variable rate does so less efficiently than a motor which is permitted to produce the same amount of work, but at a relatively constant rate.
It is therefore an object of the present invention to provide a drive mechanism for a linear peristaltic pump that results in variable pump finger speed over a stroke cycle. It is a further object of the present invention to provide a drive mechanism for a linear peristaltic pump which produces variable pump finger speed while maintaining a substantially constant torque (load) on the motor. It is yet another object of the present invention to provide a drive mechanism for a linear peristaltic pump that is relatively inexpensive to manufacture, easy to use, cost effective, and durable and reliable in its operation.